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A blazar is an active galactic nucleus whose relativistic jet is viewed close to its direction of motion. Relativistic beaming enhances the apparent radiation from approaching material and changes observed variability timescales. Its extreme brightness is therefore affected by orientation as well as by the power supplied by black-hole accretion.

A source's brightness and spectrum do not directly identify the particles responsible for every emission component. Radio through gamma-ray observations and their timing help distinguish candidate mechanisms, while TXS 0506+056 illustrates the additional information supplied by a possible association with high-energy neutrinos.

The 2018 collaboration paper associated the 22 September 2017 neutrino alert with the flaring blazar TXS 0506+056 through directional and multiwavelength observations. Its approximately $3\sigma$ evidence included trial corrections and assumed relationships between neutrino and gamma ray emission. A single association did not establish a production mechanism or explain the entire diffuse neutrino flux.^[ref]

TXS 0506+056 is a blazar studied through a gamma-ray flare associated in direction and time with the high-energy neutrino alert IceCube-170922A. The event motivated coordinated observations across the electromagnetic spectrum, connecting a variable relativistic source with a possible source of energetic neutrinos.^[ref]

An association is a statistical inference involving localization, timing and backgrounds. It does not directly image the neutrino-production site or uniquely determine the jet's particle composition. The physical interpretation must account for the measured photons and neutrino evidence together.